Got this orginal Hellfire PCB (by Toaplan) for a repair:

After some initial problem due dirty sockets of program ROMs and oxidized JAMMA edge connector which caused missing boots, I was greeted by this scenario:

Sprites were wrong, garbled and stretched all over the screen.This part of graphics is generated by the custom ASIC ‘FCU’ :

I did a reflow of it but this didn’t lead to any improvement.Probing around I found a 6116 SRAM with some data lines stuck low:

Chip failed once tested out-of-circuit:

But still no big improvements.Testing TTLs with my logic comparator I found a 74ALS169 counter @3N with bad outputs:

Its output pin 15 was almost shorted to VCC (only 7.9 Ohm measured)

Once removed, the chip failed miserably :

Finally the sprites were correctly drawn but blocky :

I noticed that, if I pressed down the ROM 9 @4E, sprites were 100% restored:

Removing the EPROM revealed oxidation on socket pins:

So I replaced it:

Installing a new socket fixed game completely.

I received this 1943: The Battle of Midway Mark II in the past days for dumping and repair :

Game is an unofficial US version of the 1943 Kai, probably an hack where airplane sprites are from 1943 Kai but the background is from 1943.I obviously dumped it and it has been already emulated in latest MAME 0.175 release:

The board was in good shape despite its age, it booted up too but crashed or freezed after few seconds of attract mode or after started a new play:

In the beginning I tought about some problem in code execution so I replaced the WORK RAM as well as suspicious ICs involved in Z80 address/data bus but this didn’t lead to any improvement.Testing the board with a logic probe, a logic comparator and a scope gave me the feeling like it was some kind of disturb on the board which afftected timing causing freezes and crashes.In the below video you can see how the HP10529A logic comparator detected a glitch pulse in the exact moment when the game crashes :

Also probing the clock signal of Z80 with a scope revealed some interferences in the waveform.

From my experience often a capacitor with increased ESR can inject disturb in the whole board since it can no longer filter ripple current.So I went to test the electrolytic capacitors in circuit with my ESR meter, they were all OK except the one @CC18 (9.2 Ohm measured against a typical value of 2 Ohm for a 47uF 16V)

Among other things, this capacitor is vital for main Z80 /RESET signal generation since it’s part of the typical circuit made along with a voltage monitor like the PST518 IC adopted on this board:

Once removed it, I had confirm of his increased ESR, besides also capacitance was half of its declared value:

Replacing the capacitor with a good one fixed the game completely.